The green thing is the circuit whose current/voltage is being measured.
In a certain way it is: it is a small pcb with two power resistors. It is a test pcb for testing purposes only.
Usually you insert the meter between a usb power port (e.g. wall wart) and a usb device to measure correct voltage and current consumption.
When you insert the test pcb instead of a usb device, the resistors will draw a certain current to load the power port. The meter will then show if the power port can sustain the nominal usb voltage (5 V) or if the voltage drops. This measurment can be used to qualify usb cables by the voltage drop along them. You put the usb cable to be qualifiied between the meter and the power port, the load at the output of the meter. Compare the voltages with and without cable to determine the voltage drop along the cable. Good cables should show only a few tens mV of voltage drop.
The black box at the end of the test pcb seems to be a fuse to preotect the meter, the resistors and last not least the power port from overcurrent.
